1. Field of the Invention
Process for using microwave irradiation to prepare solutions of polymers functionalized with acid groups.
2. Description of the Prior Art
The present invention relates to a process for using microwave radiation to prepare solutions of polymers functionalized with acid groups, to a process for preparing the solid, soluble or insoluble polymers, and also to the use of the solutions.
Many high-performance polymers, such as polyether ketones, partially fluorinated or perfluorinated polymers and polyphenylene sulfides, are highly insoluble. While this insolubility is precisely what is required for many applications, it does, however, make the processing of these polymers very considerably more difficult, or in extreme cases impossible.
The derivatives of these polymers functionalized with acid groups, such as xe2x80x94SO3H, xe2x80x94B(OH)2, xe2x80x94CO2H and xe2x80x94PO3H are, in contrast and depending on their degree of functionalization, soluble in solvents such as dimethylformamide, dimethyl sulfoxide, dimethyl-acetamide or N-methylpyrrolidone. With particularly high degrees of functionalization these polymers may also become water-soluble.
There is great demand for aqueous solutions of polymers of this type, functionalized with acid groups. Examples of the advantages of dispensing with organic solvents are cost reduction, environmental protection factors and factors relating to workforce health. It is also desirable, and particularly in the field of preparation of noble-metal catalyst materials, to avoid, or at least minimize, the use of solvents containing hetero-atoms, in particular containing chlorine, sulfur or nitrogen, since these can act as catalyst poisons. This is the reason for, for example, interest in aqueous preparations of proton-conducting polymers for producing gas diffusion electrodes for fuel cells or electrolysis units.
U.S. Pat. No. 5,453,161 discloses the preparation of polyimides derived from benzophenone-3,3xe2x80x2,4,4xe2x80x2-tetracarboxylic acid, where the reaction mixture is heated by microwave radiation. The product is not reported to be water-soluble.
JP-05 310 907 discloses a process for removing methylene chloride from polymers by using microwave radiation. There is no report that the solubility of the polymers is higher after the microwave irradiation than is the case with traditional processes for introducing heat.
Against the background of the prior art, the object was to develop a process by which polymers carrying acid groups can be prepared in a form which gives rise to greater solubility in water and in organic solvents.
Surprisingly, it has been found that polymers carrying acid groups are more soluble in water and in organic solvents when exposed to microwave radiation than when exposed to heat and pressure.
The invention provides a process for preparing aqueous, hydrous and anhydrous solutions of polymers functionalized with acid groups, which comprises using microwave radiation to supply the heat required to prepare the solution.
Even polymers whose low degree of functionalization gives them no, or only very slight, solubility, even on heating under pressure (temperature up to 175xc2x0 C., pressure up to 4 bar) can be dissolved by this process. The acid groups with which the polymers have been functionalized are preferably sulfonic acid, phosphoric acid, carboxyl and/or boric acid groups.
Possible solvents are especially water, dimethyl-acetamide, N-methylpyrrolidone, dimethyl-formamide, dimethyl sulfoxide, alcohols such as isopropanol, and also mixtures of two or more of these substances.
The polymers preferably used are polyether ketones, polyphenylene sulfides, partially fluorinated or perfluorinated aliphatic polymers or polyether sulfones, in particular those with an ion-exchange capacity (IEC) of from 0.5 to 2 mmol of acid function per g of polymer. Particular preference is given to the use of polymers of the formula 1 
which are marketed by DuPont with the tradename (copyright)Nafion. The coefficients are: Zxe2x89xa71, m=from 5 to 14, and n=100.
Unlike when heated without microwave irradiation, it has been found that especially sulfonated polyether ketones (PEKs), polyether ether ketones (PEEKs) and polyether ether ketone ketones (PEEKKs) can be dissolved with a significantly lower degree of sulfonation with microwave irradiation. No detectable molecular weight degradation takes place when the novel process is carried out. The solution result achieved here, under relatively mild conditions (low temperature and pressure), is at least comparable and in fact mostly significantly better than when heating without microwave irradiation. It is clear that the microwave radiation itself, and not the heat released by the microwave radiation, is responsible here for the good solubility performance of the polymers under these conditions.
In addition, polymers whose low degree of functionalization gives them no, or only slight, solubility in a non-aqueous solvent without microwave irradiation when heating, even under pressure, can be dissolved using microwave irradiation in N-methylpyrrolidone, dimethylacetamide, dimethyl-formamide, org. sulfoxide, such as sulfolan, or in dimethyl sulfoxide. This makes it possible for the first time for polymers with a low degree of functionalization, i.e. with a small proportion of derivatized repeat units, to be processed from solution. For example, it is possible to process sulfonated polyether ketones with a degree of sulfonation of xe2x89xa635% from NMP solution.
The invention also provides a process for obtaining the solid polymers from the microwave-irradiated solutions. After the novel process has been carried out, these polymers are insoluble in the solvent from which they were obtained.
The solutions of the polymers in water may be concentrated by evaporation to dryness. The solid polymer regained in this way may be dissolved in water by heating without microwave irradiation, or annealed to become water-insoluble, and from this condition may once again be converted into a water-soluble condition by microwave irradiation. The advantage of this surprising property is that the polymers can be transported in a soluble form without solvents, then can be dissolved without microwave irradiation and, after processing and after removing the solvent by evaporation, can be annealed to become insoluble.
No degradation of molecular weight takes place here during the microwave irradiation, as can be shown with the aid of gel permeation chromatography (GPC) or thermal field flow fractionation (TFFF). Dissolved and undissolved fractions also show no differences in molecular weights and the degree of sulfonation which they show is uniform. In the course of dissolving, therefore, no extraction of more highly sulfonated or low-molecular-weight compounds takes place.
The solutions prepared by the novel process are suitable, for example, for producing gas diffusion electrodes, fuel cells and polymer-electrolyte-stabilized platinum nanoparticles.